1. Field of the Invention
This invention relates generally to a series type voltage regulator and modulator, and in particular to a combined regulator and modulator for use in high voltage applications such as for radar and microwave power supplies.
2. Description of the Prior Art
Microwave tubes such as magnetrons, Klystrons, traveling wave tubes (TWT), and the like must usually be operated at specific peak pulse currents, within a tolerance of a few percent to hold power output constant. This is desirable from the system standpoint, because low power output reduces system performance, and high power output is wasteful of prime power, overheats components, and raises the cooling burden. For example, a supply voltage drop of only two percent will cause a wide bandwidth TWT device to lose half of its output power. In crossed-field tubes other modes may lie near the normal operating point; these usually must be avoided. In linear beam tubes gain and bandwidth shift rapidly with changes in beam current. Thus it is generally desirable to supply microwave tubes with a rippleless d.c. voltage, and in the prior art a primary control regulator is often used for this purpose. It is called a primary regulator because it is frequently placed in the primary or a.c. side of the high voltage power supply transformer. Primary regulators are slow to respond to rapid fluctuations in power line voltage and transients, and thus large and expensive passive filter components are usually required. To complement the primary regulator, a series regulator is often added on the secondary or d.c. side of the power supply, as described in Skolnick, M. I. Radar Handbook, Chapter 7.17, McGraw-Hill, Inc., New York, 1970. The prior art series regulator is typically placed between the output of the high voltage power supply and the load which is often a microwave frequency vacuum tube operating at an extremely high voltage, often in excess of 9,000 volts. Because the regulator must interact with this extremely high voltage, the conventional regulator employs one or more heavy duty vacuum tubes. A shortcoming of the conventional regulator is that, being designed to withstand extremely high voltages, it cannot respond as fast as present day semiconductor devices. Semiconductor devices are not widely used in conventional series regulators because it is very difficult to couple these delicate devices across the extremely high voltage and maintain proper biasing for safe operation of the semiconductor.
Aside from regulating the high voltage power supply to remove unwanted ripple, it is also frequently desirable to intentionally impress an amplitude modulation of the power supply current in order to induce radio frequency oscillations in the microwave tube. This process, known as modulation, is discussed in Skolnick, Radar Handbook, Supra at Ch. 7.11-7.16. The typical modulator is a separate circuit from the power supply regulator and often employs heavy duty vacuum tube devices, large capacitors, and even coupling transformers. One popular class of modulators is the active-switch modulator, also called hard-tube modulators. Active switch modulators require switching devices that can be both turned on and turned off at will, since the switching device controls both the beginning and the end of the pulse. A frequently used active switch modulator is the cathode pulser which controls the full beam power of the r.f. tube, either directly or through a coupling circuit. Because the beam power is at extremely high voltage the same coupling difficulties of regulator circuits are encountered when semiconductor devices are used. Despite these difficulties, there is a great deal of interest in using semiconductor devices for their low cost, small size and weight, low power consumption, and high operating speed. Special circuits have been developed, for example, to make SCR devices turn on and off at desired times by means of other SCRs. However, these circuits do not adequately solve the problem of inserting and biasing a wide range of delicate semiconductor devices in high voltage circuits. Furthermore, present day modulators are only capable of using a.c. type signals for modulation; more complex modulation such as digital noise and swept high frequency spot noise have not heretofore been possible.
The present invention solves the biasing problem inherent in prior art power supply regulator circuits and modulators, and provides a very responsive circuit for both regulating and modulating a high voltage power supply output. The invention is capable of applying a wide range of modulations ranging from the conventional a.c. modulation to the exotic digital noise and swept high frequency spot noise.